Aggregative mechanisms and behavior in ring-billed gulls departing from a colony

Abstract

Departures of ring-billed gulls (Larus delawarensis) from a breeding colony were significantly clumped in time and space. Mean departure directions did not correlate among successive days, but were highly correlated with wind direction. Most gulls circling about the colony did not depart. Gulls that did depart typically flew directly away from the colony and approximately half emitted distinctive "contact" calls. Significantly more gulls departed when others were soaring on nearby thermals than at other comparable times. Playback experiments showed that contact calls and calls from thermal flocks attracted other gulls. A causal model of flock formation, derived from the assumption that temporal clumping arises from social facilitation superimposed upon random departure times, was supported by (i) random fly-up times by nondeparting gulls, (ii) demonstrated attractiveness of contact calls, (iii) characteristically direct flight paths of departing gulls, and (iv) by simulations of departing gulls. Social attraction, temporal and spatial grouping, wind, and visual contact between successively departing birds all appear to facilitate aggregation within localized regions of the habitat.